Focusing method for interferometer

Abstract

A focusing method for interferometer wherein a light flux passing through a converter lens for converting the shape of wave fronts is caused to be incident on a tested surface and a testing light beam reflected by the tested surface is caused to be incident on a light receiving element capable of measuring the distribution of intensity of the light at least one-dimensionally. The tested surface is moved in the direction of its center axis until it reaches a position in which the vertex of the tested surface coincides with the focus of the converter lens and the output of the light receiving element shows a peak value, and the tested surface is further moved, from the position referred to hereinabove which serves as a reference position, in the direction of its center axis.

What is claimed is: 
     
       1. A focusing method for an interferometer comprising the steps of: passing a collimated light beam through a converter lens to convert the shape of the wave fronts of the light beam to be spherical and causing said light beam to be incident on a tested surface;   collecting the light beam which is reflected by said tested surface and substantially collimated back through said converter lens, and passing said reflected light beam through a condenser lens so as to be incident on a light receiving element capable of measuring the distribution of intensity of the light beam in at least one dimension;   moving the tested surface in the direction of its center axis until it reaches a reference position in which the vertex of the tested surface is brought into coincidence with the focus of the converter lens and the light receiving element measures an output of a peak value; and   moving the tested surface from said reference position further in the direction of its center axis until it reaches a focusing position in which the center of the radius of curvature of the spherical wave front of the incident light beam falling on the tested surface is brought into coincidence with the focus of the converter lens and the light receiving element measures an output of another peak value, wherein the distance between said reference position and said focusing position provides an approximate measure of the radius of curvature of said spherical wave front of said incident light beam.   
     
     
       2. A focusing method as claimed in claim 1, wherein the tested surface is moved in said second moving step from said reference position to the focusing position by a distance corresponding to the designed radius of the tested surface. 
     
     
       3. A focusing method as claimed in claim 1, further comprising the steps of using a reference standard formed accurately to have the radius of curvature of a reference surface and moving said reference standard from a reference position in the direction of its center axis to a focusing position while measuring the distance covered by the movement of the reference standard and to determine the relation between the distance covered by the movement of the reference standard and the radius of curvature of a reference wave front falling on the reference standard when the latter is located in the focusing position, and repeatedly performing the step described hereinabove by using a plurality of reference standards having different radii of curvature to prepare a graph indicating the relation between the distances covered by the movements of the reference standards and the radii of curvature of the reference waves to enable a focusing operation to be performed by using said graph. 
     
     
       4. A focusing method as claimed in claim 1, wherein when focusing of a plurality of tested surfaces having the same designed radius but differing from each other in the actual approximate radius of curvature is performed, focusing is performed by following the steps described in claim 1 after causing the radii of curvature of reference wave fronts falling on the tested surfaces to match a set radius of curvature which is predetermined with respect to the designed radius of the tested surfaces.